Sources of inoculum for Phytophthora ramorum in a redwood forest.

ABSTRACT Sources of inoculum were investigated for dominant hosts of Phytophthora ramorum in a redwood forest. Infected trunks, twigs, and/or leaves of bay laurel (Umbellularia californica), tanoak (Lithocarpus densiflorus), and redwood (Sequoia sempervirens) were tested in the laboratory for sporangia production. Sporangia occurred on all plant tissues with the highest percentage on bay laurel leaves and tanoak twigs. To further compare these two species, field measurements of inoculum production and infection were conducted during the rainy seasons of 2003-04 and 2004-05. Inoculum levels in throughfall rainwater and from individual infections were significantly higher for bay laurel as opposed to tanoak for both seasons. Both measurements of inoculum production from bay laurel were significantly greater during 2004-05 when rainfall extended longer into the spring, while inoculum quantities for tanoak were not significantly different between the 2 years. Tanoak twigs were more likely to be infected than bay laurel leaves in 2003-04, and equally likely to be infected in 2004-05. These results indicate that the majority of P. ramorum inoculum in redwood forest is produced from infections on bay laurel leaves. Years with extended rains pose an elevated risk for tanoak because inoculum levels are higher and infectious periods continue into late spring.

Direct measurement of strain in the posterolateral bundle of the anterior cruciate ligament.

The general objectives of the work reported in this article were to describe and validate a method for directly measuring strain in the posterolateral bundle (PLB) of the anterior cruciate ligament (ACL). The method is a procedure for gaining surgical access to the posterior fibers of the PLB through a portal incised in the joint capsule and then suturing a liquid metal strain gage (LMSG) on to these fibres. Because the incision possibly alters the load-displacement mechanics of the joint, validation included performing experiments to test the hypothesis that the incision did not significantly affect load-displacement relations. To illustrate the utility of the method, strains in both the anteromedial bundle (AMB) and PLB were measured and compared over the full range of flexion. Validation experiments revealed that the capsular incision as well as other incisions had no measurable effect on the load-displacement mechanics of the joint. Also, the PLB strain was significantly different from the AMB strain during passive flexion with a reciprocating function in load sharing evident between the two bundles.

Strain in the medial collateral ligament of the human knee under single and combined loads.

Strain within the medial collateral ligament (MCL) was measured in 13 human knee specimens to determine both the single and combined external loads most likely to cause injury. Using a load application system which allowed six degrees of freedom with flexion angle being fixed, both single loads of anterior/posterior force, medial/lateral force, varus/valgus torque, and internal/external axial torque and all pairs of these loads were applied at flexion angles of 0 degrees and 30 degrees. Liquid mercury strain gages were used to measure strain at four sites in the MCL. Two of the sites were the anterior fibers superior and inferior to the joint line and the other two were posterior of the two anterior sites. A factorial analysis revealed a significant interaction between the site experiencing the greatest strain and flexion angle. The posterior superior site experienced significantly greater strain at 0 degree flexion whereas strains was significantly greater at the anterior superior site at 30 degree flexion. Of the single moments, external axial was more damaging than valgus in that the strain developed at equivalent load was significantly greater. None of the moment-moment combinations was identified as being significantly more damaging. A similar result held for the force-moment combinations.

Strain in the anteromedial bundle of the anterior cruciate ligament under combination loading.

Strain within the anteromedial bundle (AMB) of the anterior cruciate ligament (ACL) was measured in 13 human knee specimens in order to determine the combination of external loads most likely to cause injury. Using a load application system that allowed 5 df with the flexion angle being fixed, pure loads of anterior/posterior force, medial/lateral force, varus/valgus torque, and internal/external axial torque were applied at three flexion angles: 0 degrees, 15 degrees, 30 degrees. Combined loads were applied in pairs at two flexion angles: 0 degrees and 30 degrees. Liquid mercury strain gauges were used to measure strain in the ACL. Anterior tibial force was the primary determinant of strain in the anteromedial bundle. This strain was significantly larger at 30 degrees flexion than at 0 degrees. The strain sensitivity of the AMB to medial force was approximately one-half that to pure anterior force. The effect of anterior and medial forces was additive when applied in combination. Neither pure axial torque nor pure varus/valgus torque was observed to strain significantly the AMB at any of the flexion angles investigated. However, valgus torque in combination with anterior force resulted in a significantly larger strain than pure anterior force. Internal axial torque in combination with anterior force also resulted in a larger strain than pure anterior force.

Implementation of a five degree of freedom automated system to determine knee flexibility in vitro.

This article describes an automated system designed to study the complete flexibility functions of the knee in vitro. The system allows five degrees of freedom with flexion angle being fixed, though adjustable from 0 to 45 deg. Loads corresponding to each of the five motions can be applied independently and in any combination. The effect of weight bearing on knee flexibility can also be studied by including axial force as one of the five loads. The relative motions are measured with LVDT's and RVDT's, and the loads are measured with strain gage transducers. The system is digitally controlled with a closed feedback loop, allowing for any combination of programmed loads. A control algorithm on an IBM PC/AT monitors the loads on each axis and continuously adjusts stepping motors to correctly follow programmed loads. The machine coordinate system corresponds to clinically accepted definitions of motion yet retains sequence independence for rotations. Results are presented demonstrating the repeatability of using a functional definition of axis placement to align the leg within the machine. Results are also presented demonstrating the utility of the full flexibility functions of the knee, notably in the determination of significant load interactions between anterior/posterior force and internal/external torque, and varus/valgus torque and internal/external torque.